[C++] Chap 09 - Strings

C-strings

• Array with base type char
• Plus one extra character : \0
  • Called Null character or end marker
• char s[10] : C-string variable that can hold up to 9 characters
• plus one character \0
• Typically “partially-filled” array
  • Declare large enough array to hold max-size string
  • Indicate end with null \0
• One difference : Must contain the null character
• Initialization of c-string
  • char myMessage[20]="Hi there.";
  • Need not fill the entier array
  • Initialization automatically places \0 at the end
• Can omit array-size :

char shortString[] = "abc";
cout<<sizeof(shortString); // 4 (abc\0)
char shortString2[] = {'a', 'b', 'c'};
cout<<sizeof(shortString2); //3 (abc)

• Partially filled array : un-filled part filled up with \0

char ourString[5] = "Hi";
//ourString[0] is 'H'
//ourString[1] is 'o'
//ourString[2] is '\0'
//ourString[3] is '\0'
//ourString[4] is '\0'

• To properly manipulate C-string, require library called <cstring>
• Can ONLY use = at the declaration of c-string.

char aString[10];
aString = "Hello"; //Illegal!

Assign value to c-string

• Must use library function for assignment:
• strcpy(astring, "Hello");
• No check for size

Compare two C-string

• Must use compare function
• strcmp(aString, anotherString);
• omit 0 if same else other value (lexicographical order)
• \0 is not compared (I guess)

char shortString[] = "abc";
char anotherString[] = {'a', 'b', 'c'};
cout<<strcmp(shortString, anotherString); //0

• strncpy(a, b, s); copies at most s characters of b into a
• strncat(a, b, s); concats at most s characters of b right back to a
• strncmp(a, b, s); compares at most s characters of b with a

C-string Input, Output

• Can output with insertion operator <<
• Possible because << is overloaded for C-string
• Can input with extraction operator >>
• All whitespace(black, tabs, line breaks) are skipped

char a[80], b[80];
cout<<"Enter some input:\n";
cin>>a>>b;
cout<<a<<b<<"END of OUTPUT\n";
//Do be do to you
//DobeEND of OUTPUT : whitespace skipped

• To acquire entire line with including whitespaces, must use getline
• Member function of cin
• Take two arguments : C-string variable, size of the variable

char a[80], b[5];
cin.getline(a, 80);
//Do be do to you!

cin.getline(b, 5);
//Do be do to you!

cout<<a<<"END OF OUTPUT\n";
//Do be do to you!END OF OUTPUT
cout<<b<<"END OF OUTPUT\n";
//Do bEND OF OUTPUT 
//b[4] is \0

• The reading ends when the line ends, even thought the resulting C-string may be shorter than the second argument
• Detects \n or reads until given size and substitute \n to \0
• To directly manipulate C-strings, use get

char c1, c2, c3, c4, c5;
cin.get(c1);
cin.get(c2);
cin.get(c3);
cin.get(c4);
cin.get(c5); 
//AB
//CD

cout << c1 << c2 << c3 << c4 << c5;
//c1=A, c2=B, c3='\n', c4=C, c5=D

• cin.get can be used to detect \n

char symbol;
do
{
	cin.get(symbol);
	cout<<symbol;
} while (symbol!='\n');

int number;
cin>>number; //21
char symbol;

cin>>symbol; //A
//cin.get(symbol); if inputting 21ab, number=21, symbol=a
cout<<symbol;
//when using cin.get(symbol), symbol is set to '\n'
//while cin>>symbol sets 'A'

• Unexpected \n in input.

Character-manipulating function

• Library <cctype> contains the tools
• toupper , tolower, isupper, islower , isdigit, isalnum, isspace, ispunct(punctuation mark), isprint(printing letter including space), isgraph(wo space), isctrl (control character)
• Not a member function. Workes independently

String

• string type defined in library <string>
• We can easily assign, compare and add.
• Type conversion happens(C-string to string)
• The string variable is not terminated with \0
• Extraction >> still ignores whitespace

string s1, s2;
cin >> s1;
cin >> s2; //Input : May the hair ...
//s1 : May
//s2 : the

• To read complete lines, use getline()

string line;
getline(cin, line); //caution. the first argument is cin, 
//the second is line (string variable)

• If there were leading/trailing blanks, they will be part of the string value too
• When mixing cin>>variable and getline together, there’ll be error

void newLine(){
    char symbol;
    do{
        cin.get(symbol);
    } while (symbol!='\n');
}
int main() {
    int n;
    string line;
    cin>>n;
    newLine();
    getline(cin, line); //line = '\n' if there's no newline()
}

• To access each character of string
  • lastName[i] : does not check for the illegal index
  • lastName.at(i) : checks and omits error if out of range
• str.length() function returns the length of the string object

Member functions of the standard class string

• Constructors
  • string str (default, creates empty object str)
  • string str("astring"); : Creates string object with c-string
  • string str(aString); : Copy constructor
• Element access
  • str[i] / str.at(i) checks the illegal index or not
• Assignment
  • str1 = str2 assigns
  • str1 += str2 / str1+str2 concats
  • str.empty() : returns true if empty
  • str.insert(pos, str2)
  • str.remove(pos, length)
• Comparisons
  • ==, != : bool
  • <, >, <=, >= : compare with lexicographical order
  • str.find(str1) : find index of first occurrence of str1 in str
  • str.find_first_of(str1, pos): find index of first instance of str1 in str (”Hello world”, “abcde”); → 1

Conversion between string and c-string

• Automatic conversion from C-string to string but not vice versa

char aCstring[] = "This is my C-string.";
string stringVar;
stringVar = aCstring; //Legal
strcpy(aCstring, stringVariable); //Illegal
strcpy(aCstring, stringVariable.c_str()); //Legal